1. Field of the Invention
The present invention relates generally to powertrain systems and, more particularly, to a hybrid powertrain system employing at least two prime movers and a dual independent countershaft transmission.
2. Description of the Related Art
Hybrid vehicle powertrain systems employing two or more prime movers to drive the vehicle are well known in the art. Previous hybrid powertrain systems have utilized an internal combustion engine that is strategically operated in combination with an electric motor to provide driving torque to the wheels of the vehicle.
So-called “parallel hybrid” powertrain systems have been developed that commonly interface the electric motor in line between the vehicle engine and the transmission. In one known parallel hybrid system, the motor rotor is coupled directly to the engine output shaft. In this type of parallel hybrid powertrain, a main clutch must be operated conventionally to disengage the engine from the transmission. Such use of a conventional clutch precludes the ability to powershift the transmission, i.e., shift the transmission while retaining drive torque on the transmission output shaft, which is preferable to maximize driver comfort and facilitate smooth, rapid acceleration of the vehicle. Another limitation of this type of parallel hybrid powertrain system is that it requires a high torque, low speed electric motor to match the relatively low speed of the engine.
In another known parallel hybrid powertrain system, the torque output produced from an internal combustion engine and the torque output produced from an electric motor are combined in a torque composition mechanism, such as a planetary gear train. The combined torque output is then transmitted, via a conventional transmission, to the drive wheels. A limitation of this type of powertrain system is that the output torque of the engine and motor must be precisely controlled to balance one another during steady state operation. Another limitation of this type of powertrain system is that regenerative braking of the vehicle requires complex hardware and control systems.